In a conventional W-CDMA (Wideband-Code Division Multiple Access) mobile communication system, a downlink high-speed packet transmission scheme (HSDPA: High Speed Downlink Packet Access) is being developed under which a high-speed, large-capacity downlink channel is shared among a plurality of communication terminal apparatuses and packet data is transmitted from a base station apparatus to a communication terminal apparatus at high speed.
Here, HSDPA in a W-CDMA system will be explained briefly. A communication terminal apparatus measures a reception CIR (Carrier to Interference Ratio) and reports information (e.g., CQI: Channel Quality Indicator) indicating a downlink channel condition to a base station apparatus based on the measured CIR. The base station apparatus determines a communication terminal apparatus to which packet data is to be sent (transmission destination apparatus) based on CQIs reported from the respective communication terminal apparatuses. This is called “scheduling.” Furthermore, the base station apparatus determines according to what modulation scheme and what coding rate (MCS: Modulation and Coding Scheme) packet data to be sent to the transmission destination apparatus should be processed based on the downlink channel condition indicated by the CQI. This is called “MCS assignment.” The base station apparatus sends packet data to the determined transmission destination apparatus according to the determined MCS.
As a specific example of MCS assignment, suppose a case where a fading variation as shown in FIG. 1 occurs. FIG. 1 illustrates a time variation of reception power due to fading. Suppose, the horizontal axis shows a time, the vertical axis shows reception power, and the reception power becomes a maximum at t1 and the reception power becomes a minimum at t2. It is decided that the propagation path is in a good condition at t1 and a high MCS level (e.g., 16QAM, coding rate ¾) is assigned. On the other hand, it is decided that the propagation path is in a poor condition at t2 and a low MCS level (e.g., QPSK, coding rate ¼) is assigned. That is, when the propagation path is in a good condition, high-speed transmission is possible.
Therefore, if a communication terminal apparatus assigned a high MCS level is determined as the transmission destination, it is possible to send a large amount of data in a short time and thereby improve throughput of the system.
Furthermore, a conventional W-CDMA system maintains reception quality per bit by controlling transmit power, while the HSDPA can maintain reception quality per bit by controlling the MCS as described above.
The above described HSDPA is a technology based on the premise that it is used for a W-CDMA system, and the application of the HSDPA technology to an OFDM (Orthogonal Frequency Division Multiplexing) communication apparatus which is a promising next-generation communication scheme is under study. The following are examples of the HSDPA technology applied to OFDM.